The present invention relates to a Bi2O3xe2x80x94ZnOxe2x80x94Nb2O5 (BZN) ceramic material, and especially to a BZN ceramic material having a low sintering temperature by adding a flux therein.
Microwave ceramic materials in the microwave frequency, f, ranged from 1 to 12 GHz are used as dielectric resonators, e.g. bandpass or bandstop filters and oscillator stabilizer devices. These devices are becoming increasingly important with the continued development of microwave integrated circuitry, microwave telecommunications, and satellite broadcasting systems.
BaOxe2x80x94RE2O3xe2x80x94TiO2 is one system of ceramic materials being developed for microwave applications, wherein RE2O3 is a rare earth oxide, e.g. La2O3, Nd2O3, Sm2O3, etc. See for example: (1) Journal of the American Ceramic Society, K. Wakino, et al., 67(4), 1984, pp.278-281 and (2) Am. Ceram. Soc. Bull., S. Nishigaki, 66(9), 1987, pp.1405-1410. These ceramic materials have high dielectric constant (∈r≈70xcx9c90) and low temperature resonant coefficient (xcfx84f less than 10 ppm/xc2x0 C.). BaOxe2x80x94MgOxe2x80x94Ta2O5 is another ceramic material system being developed. This ceramic material has high dielectric loss quality (Qxc2x7f≈150000). See for example: J. Appl. Phys., M. Furuya, 85(2), 1084 (1999). Although these ceramic materials have excellent microwave dielectric characteristics, they need to be densified at a higher sintering temperature above 1300xc2x0 C. Therefore, the processes for manufacturing the electronic devices need to be further modified. In order to avoid resonance interference and dielectric loss, the electrodes used in the multilayer structure should exhibit a high conductivity. Although Ag-based internal electrode is the best choice, the melting point of Ag is quite low (Tm=961xc2x0 C.). If the sintering temperature of the ceramic materials is too high, Ag-based internal electrode can not be utilized. Therefore, the point of present invention is to provide a low-temperature cofired ceramic material with good microwave properties.
The reports of recent researches for lowering the sintering temperature of the ceramics can be divided as follows:
(I) A first method to lower the sintering temperature of microwave ceramics such as BaOxe2x80x94RE2O3xe2x80x94TiO2 from 1300xc2x0 C. to 950xc2x0 C. is done by adding a lot of glass phases (10 wt %xcx9c30 wt %) inside. However, adding glass phases will lower the dielectric constant from 90 to 20. Moreover, the sintering temperature (950xc2x0 C.) is not low enough for the low-temperature cofired ceramic (LTCC) applications. See for example: (1) U.S. Pat. No. 4,504,339 and (2) Journal of The American Ceramic Society, T. Takada, 77(7), 1994, pp.1909-1916.
(II) A second method to lower the sintering temperature of microwave ceramics is made by using a glassy-phased dielectric ceramics, which is formed by melting all oxides and then quick cooling the mixture. The sintering temperature can be lowered to 850xc2x0 C. but the dielectric constant is only 7xcx9c13. See for example: (1) U.S. Pat. No. 3,656,984 and (2) xe2x80x9cMaterials Compatibility and Cosintering Aspects of Shrinkage Control in Low-Temperature Cofired Ceramic Packages, H. Sawhill, pp.307-319, Advanced in Ceramics, Vol. 26, xe2x80x9cCeramic Substrates and Packages for Electronic Applicationsxe2x80x9d (American Ceramic Society, Westerville, Ohio, 1990).
(III) As disclosed in U.S. Pat. No. 5,449,652, a new dielectric ceramic material, Bi2O3xe2x80x94ZnOxe2x80x94Nb2O5 (or Bi2xe2x88x92x(Zn2/3Nb4/3)O7xe2x88x923x/2, 0xe2x89xa6xxe2x89xa60.67, BZN), is evolved. By improving the composition and forming method, the sintering temperature of BZN ceramics is lowered to 920xc2x0 C. and the microwave dielectric properties is excellent [∈r≈75xc2x7140, Qxc2x7f greater than 5000 (at 1xcx9c5 GHz)]. See for example: (1) xe2x80x9cPhase Structure and Dielectric Properties of Bi2O3xe2x80x94ZnOxe2x80x94Nb2O5-based Dielectric Ceramicsxe2x80x9d, D. Liu, et al., J. Am. Ceram. Soc., 76(8), 1995,pp.2129-2132, and (2) J. Am. Ceram. Soc., M. F. Yan, et al., 73(4), 1987, pp.1106-1107. (3) J. Mater. Res. 5, 1990, pp. 1752-1762. Although there are many developments done to modify BZN, the sintering temperatures of BZN ceramics in the conventional researches are all above 900xc2x0 C. and most of the dielectric properties were measured at a frequency below 10 MHz.
The reports for the dielectric properties of BZN ceramics at a high frequency in the GHz range are still not too much. European Patent No.558319A discloses that after the xBiO1.5xe2x88x92yZnOzNbO2.5, wherein 0.41 less than x less than 0.51, 0.19 less than y less than 0.3, and 0.25 less than z less than 0.345, is sintered at 925xc2x0 C., the dielectric properties at 2xcx9c4 GHz are ∈r=89xcx9c133, Q=40xcx9c310, and xcfx84f=110xcx9c120 ppm/xc2x0 C. Others try to improve the sintering temperature of BZN by replacing a small amount of Bi, Zn, and Nb elements (usually less than 20%) with Ca, Mg, Co, Sn, Ti, etc. or changing the manufacturing method of BZN. As an example disclosed in U.S. Pat. No. 5,449,652, by mixing ZnO and Nb2O5 to form ZnNb2O6 at first and sintering ZnNb2O6 with Bi2O3, ZnO, and CaCO3 at 950xcx9c1100xc2x0 C., BZN ceramics can be formed with the dielectric properties of ∈r=90xcx9c110, ff less than 100 ppm/xc2x0 C., tan xcex4(at 100 KHz) less than 0.0002, Qxc2x7f(at 1xc2x75 GHz) greater than 5000. Although BZN is an excellent low-temperature microwave ceramic material, the sintering temperature still has to be over 925xc2x0 C.
In order to lower sintering temperature, many fluxes, such as Li2CO3, B2O3, LiF, PbO and CuO, are added. See for example: U.S. Pat. No. 5,433,917 xe2x80x9cPZT Ceramic compositions having reduced sintering temperatures and process for producing samexe2x80x9d. Unfortunately, although these fluxes can be added in BaTiO3 or piezoelectric ceramics (PZT), they can not be added in BZN systems since the dielectric properties will be significantly affected.
It is therefore attempted by the applicant to develop a novel fluxed BZN dielectric ceramic materials having sintering temperature below 900 to overcome the aforementioned limitations and difficulties encountered with the prior art. Additionally, excellent microwave properties, i.e., Q (quality factor) and ∈ (dielectric constant) can be obtained at such a low sintering temperature as 800xcx9c850xc2x0 C.
It is an object of the present invention to provide a dielectric ceramic material which is mainly made by Bi2O3xe2x80x94ZnOxe2x80x94Nb2O5 (BZN). By adding a special flux in BZN, the sintering temperature is lowered and the dielectric properties still maintain excellent.
It is another object of the present invention to provide a method for forming a dielectric ceramic material by adding a flux. The flux is formed by mixing and melting the mixture oxides according to the eutectic composition of the oxides.
It is a further object of the present invention to provide a dielectric ceramic material which is formed by directly mixing, calcining and sintering Bi2O3, ZnO and Nb2O5 or by directly mixing, calcining and sintering Bi(CH3COO)3, Zn(CH3COO)2 and Nb(OC2H5)5.
The dielectric ceramic material of the present invention includes a mixture represented by the formula of Bi2xe2x88x92x(Zn2/3Nb4/3)O7xe2x88x923x/2 (BZN), 0xe2x89xa6xxe2x89xa60.67, and a flux having an eutectic composition for lowering the sintering temperature of the dielectric ceramics from 950xcx9c1100xc2x0 C. to 800xcx9c850xc2x0 C.
According to the present invention, the dielectric material has a dielectric constant higher than 45 (∈r greater than 45) and a quality factor larger than 1200 (Qxc2x7f greater than 4500) at 3.5 GHz.
The fluxes used in the present invention include BaCO3xe2x80x942.5CuO powder and 0.15CuOxe2x80x940.85MoO3 powders melted at about 1200xc2x0 C., 900xc2x0 C.
According to the present invention, the flux is added into the mixture with the addition of 0.1xc2x710 wt %, preferably 0.5xc2x75 wt %.
According to the present invention, BZN is formed from Bi2O3, ZnO, Nb2O5 calcined at 850xc2x0 C., or from Bi(CH3COO)3, Zn(CH3COO)2, and Nb(OC2H5)5 by a sol-gel method.
The method of forming a dielectric ceramic material includes (a) preparing a mixture represented by the formula of Bi2xe2x88x92x(Zn2/3Nb4/3)O7xe2x88x923x/2(BZN), 0xe2x89xa6xxe2x89xa60.6, (c) sintering the mixture at 850xc2x0 C. to obtain the dielectric ceramic material.
The present invention may best be understood through the following description with reference to the accompanying drawings, in which: